PROJECT SUMMARY This project will test the hypothesis that non-canonical activities of telomerase reverse transcriptase (TERT) promote calcification in calcific aortic valve disease (CAVD) by inducing the osteogenic reprogramming of valve interstitial cells (VICs). Premise for this has been established from our preliminary data as well as from the work of others. First, ectopic expression of TERT in human mesenchymal stem cells primes these cells to differentiate down the osteogenic lineage. Second, multiple studies have shown that TERT exerts non-canoni- cal functions that act to induce gene expression, including activation of osteogenic genes; these activities are entirely separate from TERT?s telomere-extending functions. Third, our preliminary data shows TERT is highly expressed in CAVD valve tissues compared to healthy valves, is present in VICs isolated from CAVD valves, and is upregulated in healthy VICs that are cultured under conditions that promote osteogenic differentiation. Fourth, genetic deletion of TERT inhibits calcification in an in vitro model, and knockdown of TERT reduces levels of the osteogenic transcription factor RUNX2. Lastly, we provide evidence that suggests that STAT5 may be involved in the TERT-mediated activation of osteogenic reprogramming. Our data identify a new mech- anism driving CAVD pathogenesis. The objective of this proposal is to define TERT?s contribution to CAVD through its non-canonical actions. Specific Aim 1 will test the hypothesize that TERT is required for inducing the transition of quiescent VICs into calcifying VICs. We expect that genetic deletion of TERT protects against valvular calcification by preventing the phenotypic switch of a quiescent healthy VIC into an osteogenic calcify- ing VIC. Specific Aim 2 will test the hypothesis that STAT5 induces TERT expression, and together STAT5 and TERT cooperate to induce transcription of osteogenic genes. We expect the completion of these complemen- tary studies will identify a novel mechanism regulating CAVD pathogenesis, and will highlight a novel non-ca- nonical role for TERT in ectopic calcification.